Lyon County, Iowa · Rock Rapids
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Lyon County averages 80 nights a year at 20°F or colder, mostly November to April, and 43% of its homes predate 1960.
Basements flood here mostly in June. Lyon County averages 6.4 days a year with an inch of rain or more.
Sewage backups in Lyon County spike on heavy-rain days, about 6.4 a year, when sewer mains fill up and water finds the lowest drain.
Roof leaks here come from summer downpours and, with about 97 freeze-thaw days a year, from winter ice dams. Most show up as ceiling stains in the room below.
26% of occupied homes in Lyon County heat mainly with wood or propane, above the Iowa county median of 22.1%.
With 21 muggy nights a year, mold in Lyon County can appear within days of a leak. Dry first, then clean.
Local risk profile
About 12,162 people live in Lyon County, 588 square miles in northwest Iowa, in roughly 4,870 homes. Besides Rock Rapids, the county seat, the biggest communities are Larchwood, George and Inwood. Half the homes here were built before 1967, half after.
47/100 Water Damage Risk Index: Elevated Lyon County ranks 59 of 99 Iowa counties. See the statewide ranking and method.The three factors where Lyon County ranks highest compared with other Iowa counties.
Lyon County gets more hard-freeze nights than most of Iowa. From 2015–2024 it averaged 80 nights a year at 20°F or colder, the range where pipes in exterior walls, crawl spaces and unheated garages freeze, and 158 nights at or below 32°F. That ranks 4th of 99 counties.
The longest cold spell was straight 32 days without the temperature rising above freezing, ending in March 2019. The coldest night reached about -25°F on December 18, 2016. Burst pipes usually show up when a spell like that breaks and the ice thaws.
Expect the hardest freezes in January, inside a season that runs November to April. That's when an outdoor spigot left connected to a hose, or a pipe in a crawl space, is most likely to split.
Freezing nights typically start around Oct 12 and end around Apr 25.
Lyon gets fewer downpours than most Iowa counties, but they still fill basements. Of roughly 30.9 inches of yearly precipitation, the damage comes from the 6.4 days a year with an inch or more, ranked 78th of 99. June, August and September see the most. The single wettest day in the record: 4.57 inches, June 21, 2018. The worst three-day stretch dropped 8.11 inches, ending June 23, 2024, the kind of soaking that overwhelms sump pumps and storm sewers.
About 97 days a year swing from below freezing at night to above freezing by day. Those freeze-thaw cycles feed ice dams on roofs, open cracks in foundations and driveways, and stress pipes that are already marginal.
Housing
Roughly 65% of the county's housing was built before 1980. That puts Lyon among the newer housing stocks of Iowa's 99 counties.
18% of homes were built in 2000 or later, more than in most of Iowa. Newer plumbing rarely corrodes, so damage here tends to come from appliance supply lines, water heaters, sump pumps and storms.
Beyond water
Lyon County averages 21 warm, muggy nights a year (lows of 65°F or more) and gets the most rain in June. The EPA recommends keeping indoor humidity below 60%, ideally 30 to 50%. Basements can pass 60% on their own in summer, so a dehumidifier matters even without a leak, and anything wet should be dried within 24 to 48 hours.
Mold remediation: what to expectHeating fuel matters for fire risk. In Lyon County it's 40% utility gas, 28% electric, 26% propane and under 1% wood. Propane heat is more common here than the state median. Have chimneys swept and fuel-burning appliances serviced each fall, and keep three feet clear around stoves and space heaters.
Fire and smoke cleanup: first stepsCoverage
Calls connect to restoration companies that work across Lyon County. The largest communities by population:
Population from ACS 2020-2024 5-year estimates. Townships and unincorporated areas are covered too.
Nearby
Osceola gets more pipe-freeze nights than Lyon. All of its neighbors see more heavy-rain days. Osceola County has the oldest housing nearby, with 82% of homes built before 1980.
By the numbers
| Factor | Lyon | Iowa median | Rank |
|---|---|---|---|
| Homes built before 1960 | 43% | 48% | 66 of 99 |
| Homes built before 1980 | 65% | 73% | 73 of 99 |
| Homes built 2000 or later | 18% | 13% | 26 of 99 |
| Manufactured or mobile homes | 1.1% | 3.2% | 93 of 99 |
| Vacant homes | 8% | 9.5% | 73 of 99 |
| Wood or propane heat | 26% | 22% | 38 of 99 |
| Nights at 20°F or colder | 80/yr | 62/yr | 4 of 99 |
| Days with 1 inch+ of rain | 6.4/yr | 7.1/yr | 78 of 99 |
| Freeze-thaw days | 97/yr | 94/yr | 11 of 99 |
| Muggy nights (65°F+) | 21/yr | 29/yr | 79 of 99 |
Rank 1 is the highest value. Bars run from the lowest to the highest Iowa county; the tick marks the median; orange means higher water damage risk than the median, green means lower.
Questions
Peak risk months in Lyon County: freezing from November to April, heavy rain in June, August and September.
Sources: U.S. Census Bureau, ACS 2024 5-year estimates (2020-2024); weather from NOAA nClimGrid-Daily county averages (EpiNOAA), 2015–2024, averaged across the county; county boundaries from Census TIGER. Reviewed 2026-10-21. Methodology